The primary goal of this proposal is to determine the precise effect of the lineal red cell density in capillaries (droc) on diffusive gas transport in tissue. Capillary lineal red cell density, an index of capillary tube hematocrit, can vary rapidly over a 3-fold range under different conditions. However, its precise role in gas exchange is presently unclear. Studies on amphibian skin will determine: 1) the dependence of diffusive gas flux across the epidermis on droc at different epidermal thicknesses, 2)the heterogeneity in red cell spacing within capillaries and the effects of this heterogeneity on diffusive gas transport, and 3) the effect on tissue diffusing capacity of replacing red cells with a stroma-free hemoglobin solution. This will be accomplished by testing the following hypotheses on frog skin: 1) Changes in droc affect the tissue diffusing capacity to CO (DtCO) more at low droc than at high droc. 2) As epidermal thickness increases, changes in droc have less effect on DtCO. 3) The heterogeneity of capillary red cell spacing is an important determinant of DtCO. 4) Replacement of red cells with a stroma-free hemoglobin solution augments D,CO. 5) As epidermal thickness increases, replacing red cells with a stroma-free hemoglobin solution has less effect on DtCO. 6) Mathematical models incorporating realistic geometries and solved by finite element analysis can describe diffusive gas flux across amphibian skin. All experiments will be performed on anesthetized (urethane) amphibians. CO flux will be measured across a small patch of skin by mass spectrometry for calculation of DtCO. Simultaneously, the following microcirculatory parameters will be measured on the same region of skin with fluorescent video microscopy: droc, capillary cell spacing, perfused capillary density, red cell velocity and capillary volume. In testing Hypotheses 1-3, ~~ will be altered by hemodilution and hemoconcentration. In testing Hypotheses 4 and 5, red cells will be replaced by a stroma-free solution of human hemoglobin cross- linked between the alpha chains. This hemoglobin has been used experimentally as a blood substitute. Different amphibians with a variety of epidermal thicknesses will be tested and compared. This investigation of amphibian skin will provide unique information helpful to the understanding of basic issues in mammalian gas exchange that have not been experimentally accessible because of the complexity of most tissues. The results of this research will lead to a better understanding of 1) the significance of capillary hematocrit and the very large and rapid changes that occur in this parameter, and 2) the precise effects of blood substitutes on diffusive gas transport.